In the present thesis clusters of nucleic acid bases and water are used as model systems of
cellular DNA to investigate UV induced radiation damage in the gas phase. A new experimental
system has been designed and commissioned to perform rnultiphoton ionization experiments on
hydrogen bonded clusters of the nucleic acid bases: adenine, thymine and uracil, as well as on
the related chromciphores: 5-fluorouracil and hypoxanthine. As the result of pulsed nanosecond
laser irradiation in the wavelength range of 220-230 nm. cluster, monomer and fragment ions
were detected using a time-of-flight mass spectrometer. Possible multiphoton ionization
pathways of clustered and isolated molecules including the role played by short (singlet) and
long-lived (triplet) excited states as well as excited state tautomeric transitions are discussed.
Signal intensities as a function of laser pulse fluence were measured. The slope of a logarithmic
plot of the signal intensity versus fluence yields the so-called 'photon order' and provides
information on the number of photons leading to the production of cluster, monomer and
fragment ions. The results indicate two-photon ionization for uracil and thymine in dry molecular
beams (i.e. photon orders equal to 1.9±0.2 and 1.4±O.1, respectively). An interesting result in the
form of photon order equal to 3.0±0.5 has been measured for adenine in a dry molecular beam
irradiated with an unfocused laser beam. This suggests a three photon ionization process,
however further experimental work is required in order to exclude factors related to stability of the
laser beam spatial profile.
Hydrated cluster ions with up to 7 water molecules attached to a single uracil and 4 water
molecules attached to adenine were unambiguously identified after adding water vapour to the
molecular beam source. A maximum of 7 water molecules attached to adenine-uracil base pairs
were detected. Possible ionization pathways of such hydrated clusters mediated by excited
states tautomeric transitions are discussed. On the basis of photon orders measured under
hydrated conditions, it is proposed that sequential MPI processes with 2, 3, and 4 photon
absorption lead to production of cluster ions, cluster fragment ions (including protonated
monomers), and molecular fragment ions, respectively.